The overall objective of the proposed project is to investigate and clarify the pharmacological and physiological interrelationships between calcium and the secretion and effects of calcemic and gastrointestinal (GI) hormones. The goal is to relate the findings to calcium homeostasis and gastrointestinal function in health and disease. Primary emphasis will be placed on secretion, synthesis and actions of thyrocalcitonin (TCT) and gastrin, but studies will also involve parathyroid hormone (PTH) and several other gastrointestinal homonal peptides (e.g., cholecystokinin and somatostatin). Problems to be investigated include: 1. In vivo in the pig: a) Effects of various GI hormones on secretion of TCT and PTH and the mechanism involved. b) Possible physiological significance of effects of PTH and TCT on antral gastrin secretion and the mechanisms involved. 2. In vivo in the rat: a) Development of an immunoassay for rat PTH and its use in conjunction with that for rat TCT to explore potential GI factors affecting release of calcemic hormones during feeding - esp. in suckling baby rats. b) Examination of possible detrimental skeletal consequences of TCT removal in early life in the rat. 3. Evaluation of secretion of rat TCT in vitro to elucidate mechanisms involved in biosynthesis and release of TCt and to study putative TCT secretagogues. 4. Application of radioimmunoassays and immunocytochemical procedures for cAMP and cGMP to investigate possible mechanisms by which calcium, calcemic hormones and GI hormones interact with one another.